Squish height definition question

[mskala]

Hi,
For JE pistons, they specify 0.040" clearance to the head. Note that these are
high dome. In measuring with solid solder, the thickness is only squished on the
very steep sides, as in the pic. I'm seeing 0.035" thickness minimum where the
arrows are.

So, the question is, do we really care specifically about the clearance perpendicular
(my 0.035), or do you consider it vertically, i.e. in the direction of the rod. Because
the angle there is about 60 degrees, the head is at least 0.050 away from the piston vertically.


Thanks

[rennzeit]

Good question and interesting thought considering the direction not just magnitude. Looking forward to the experts answer.
I'm sure they'll all agree

[afterburn 549]

In real life, flat pistons are not usually the fair.
So to contribute here, I have always shot for .030 at the sides to help with all the benefits associated with this menu.
That seems to be the standard number in the race industry today .( .030)
Power is lost the further from the spark area as the gap between piston and head starts to enlarge.
The piston starts for BDC and the squish keeps the pressure area tighter and more usable.
To to make up for the loss, the piston and head should be tighter at the sides to take advantage of the explosion if you will.

[mskala]

Thanks for the input. I did eventually find Bruce Anderson's line in his book stating 0.035".

To increase my clearance I would need another 0.25mm shim height, at the expense
of about 0.3 compression ratio reduction. Or, I could maybe cut 0.004" from the side of
the dome without reducing compression ratio hardly at all.

[afterburn 549]

I am not quite understanding what all what was done or what you are doing to your engine.
Why would you need to increase your clarence? something must have been machined?
Squish has little to do with your C/R ratio.
What do you have for squish right now?

[mskala]

Measurements I have been taking are with new heads I'm installing. With old (wrong) heads I had probably 0.040 clearance if I remember. New heads have definitely had at least 0.010 machined off.

[afterburn 549]

OK, Now i get it.
There are lots of ways to fix stuff.
Each has its own merits.
Generically speaking some heads designs will tighten the squish with milling, others (flatter more shallow chambers) material has to come off the bottom to get the piston closer to the top.
Personally, I like the .030 number.
Anything more than that is left on the table .
I have even gone .028 on big old Harleys .
No problems .

[mskala]

Bruce Anderson was talking these types of pistons


Where mine looks like this



Also I have the short stroke.

[afterburn 549]

I am not the end all of knowledge here .
What I think I know (pretty sure) is that piston wants a close squish at the edges. ( .030 )thou
How you want to attain that is still up to you.
The most EZ way is deck the case both sides.
However , spinning some off the bottom of all six cylinders will be better for the next owner...
I am not strictly a Porsche person. I would ask a shop that specializes in Porsche for their advice .
( Not your local machine shop)
You could PM john Walker, and a few other Masters on here and see what they offer you. ( I would)

[Trackrash]

Maybe someone can verify what the minimum piston to head clearance should be?

I have read .035". J&E recommends .040".

I am talking about the edge of the piston compared to the top of the cylinder.

Any thoughts? I don't see a clear consensus in this thread. BTW it's an SC motor with 95mm cylinders and J&E 9.5 to 1 pistons running Webers with the cams TBD.

[afterburn 549]

Like I said .030 works for everything i have done

[Trackrash]

Thanks.

I used .035" on my last build but it was with Mahle pistons.

J&Es have that lip at the edge of the piston, I wasn't sure if that would make a difference.